"Professional party-goers" is how Freddie Fellowes describes the revellers at this weekend’s Secret Garden Party.

“They’re people who aren’t there just to be spectators,” says the 36-year-old who founded the event in Cambridgeshire 12 years ago. “That’s why we call ourselves a party, not a festival: we’re trying to do something different, [not just] seeing how much production you can pile on to Arcade Fire’s headline performance.”

Fellowes’s rejection of the traditional gig-in-a-field format means the SGP gets a Marmite reaction: “People either think we are completely up our own arses and are crap, or they think it sounds really fun.”

It’s definitely always a picture editor’s dream — full of beautiful young ’uns either in fancy dress, bikinis or the buff, letting loose and getting muddy.

I’ve met Fellowes near his office in Islington. He’s the Old Etonian son of a baron and sounds like it, but looks more like a roadie, just with floppy hair. He’s wearing Ray Bans, and a T-shirt with the slogan “When fashion attacks!” and a picture of a bear looming over models.

He tells me he started the SGP “by accident”: “I was helping someone employed by Red Bull to find venues for their events. My Dad’s a farmer — so I asked him if there was anywhere on the farm to do it. Where he suggested was so wonderful, I thought it was ridiculous to pass it on.” Twelve years later, Fellowes says he has “the best job in the world.”

Beyond all the fun, Fellowes has a more serious message, though. He’s given this interview to promote the work of CALM — the Campaign Against Living Miserably, a suicide prevention charity that the SGP supports. He has picked this cause carefully: “With all the goodwill in the world, the RSPCA doesn’t need our money — and they’d spend it all on marketing anyway. Ditto to Oxfam.”

He complains that too many of his rivals pick “green” causes: “It slightly makes me twitch when I see some of the larger, more established festivals spending all this money on raising awareness about pollution and global warming — it’s a bit like raising awareness of Catholicism to a bunch of monks. These are people who are painfully aware of these concerns. That’s not really CSR [Corporate Social Responsibility], it’s just w***ing off.”

Instead, the SGP wants to address the taboo of suicide — the biggest killer of young British men. “It’s part of all the ‘peace, love and happiness’ we’re trying to spread, it’s a natural extension of why this is important. This really isn’t hippy shit that we’re spouting here, it is — we believe — true and important.”

But how can they tackle mental health within the festival? “There’s two sides,” says Fellowes. “There’s the rather painfully obvious: people will indulge at a festival, whatever their chosen poison or weapon of choice is. Law of averages, there are going to be some unhappy souls who try to pour balm on their soul in whatever form, who might overcook it.” These people get “tailor-made care and attention”.

Then there’s CALM, for more long-term problems. During the weekend, the charity offers free tea and toast in return for festival-goers writing down or talking about something that’s bothering them.

Fellowes has a particular reason for supporting CALM’s work. In May 2013, the SGP’s “greatest fan” — Vito Neo Reargo — killed himself. Vito was the life and soul of the Secret Garden Party. Fellowes describes Vito as dressing “like the rave harlequin of your dreams”; on a Facebook tribute page, his accessories include a toy snake worn as a scarf and playing card sunglasses. He also used to carry a sign that said “This is a serious party” — which has now been adopted as the SGP’s motto. There’s a memorial for him in the garden — a stone boulder cut in half — and last year, Vito’s ashes were put in rockets and let off as part of the fireworks display on the last night: “I knew he’d love a Hunter S Thompson-style send-off.”

This wasn’t Fellowes’s first experience of suicide. One of his first girlfriends died in a suspected suicide a year after they’d been together. She was just 19. “That probably ranks as one of my top three least favourite funerals I’ve been to.” Another girlfriend’s brother killed himself, as did his wife’s father. “There is no shortage of it.”

Fellowes believes that people who kill themselves are usually trying to exercise control in the last way they feel able: “This isn’t people making a mistake or anything — nine times out of 10, this is a rational, well-thought out decision by people who really do know what they are doing. That’s what’s so heartbreaking about it. This is — rationally for them —the only solution for where they are right now. I think it’s a duty of all of us to point out that isn’t the only solution — there is another way.”

His work with CALM is part of what Fellowes calls “the ethos and morality” of the event. “It’s not particularly radical or anti-capitalist — it’s just trying to be as nice as possible.”

That also means no charges for parking, camping and programmes, no sponsorship and no visible branding: “Nothing bursts your bubble — you’re not reminded about what mobile plan you’re on, or whether your bank’s good for you or not. No one’s pushing that down your throat.”

As the interview wraps up, we turn to the aftermath. How long does the clean-up take afterwards? “It depends what toys you give them to play with.” He recalls one nightmarish year when revellers had a straw fight. “Clearing up tonnes of loose straw from the field traumatised one of my staff.”

Fellowes himself has a “high tolerance” of the madness, the mud baths and the nudity. He says nothing that happens at the party shocks him. So what would shock Disgusted of Tunbridge Wells, then? There’s a snort. “How long have you got?”